1. Field of the Invention
The present invention relates to a wireless communication apparatus including a slit antenna that is used for wireless communication.
2. Description of the Related Art
Wireless communication apparatuses such as a mobile telephone; a personal computer incorporating a mobile telephone, a Bluetooth®, a wireless LAN, a UWB (Ultra Wide Band) and others for performing the function of wireless communication; a PDA (Personal Digital Assistance); a digital still camera; a camcorder; consumer electric appliances and the like have been known. In those wireless communication apparatuses metals, such as magnesium (Mg), aluminum (Al) or other metals are used for a part or the whole of a housing in order to improve the strength thereof and to make outer appearance excellent in design. For performing the wireless communication function, an antenna is indispensable; however, the antenna may greatly lose the sensitivity thereof when located near metals or put into a metal-covered housing.
Therefore, conventionally, an antenna has been disposed outside a housing, or a part of the housing near the antenna has been made of plastics instead of metals in both the cases where the antenna was inside the housing and where the antenna was outside the housing.
However, placing an antenna outside the housing is not preferable in view of being an obstacle to achieving miniaturization of the apparatus. Also, when a part of the housing near an antenna is made of a plastic or the like instead of a metal, such problems as decreasing the strength of the housing and decreasing the degree of freedom in design may occur. Therefore, some deterioration in sensitivity may have been allowed in exchange for putting an antenna inside the housing.
In order to solve the above problems, there is known a slit antenna provided on the surface of a metal housing, which has the length equal to a half of a wave length (λ/2) of a receiving radio wave and which is covered by a high permeability member.
FIG. 1 is an explanatory view of an example of the structure of the above described slit antenna. The conventional slit antenna has a rectangular slit 21, the length of a long side LT of which is a half of a wave length (λ/2); and a feeder 22 is connected to a feeding point 23 and a ground point 24 provided at the center of the metal portion of the long side of the slit. As the feeder 22, conventionally, an unbalanced cable such as a coaxial cable is used. At that time, since the slit 21 functions similarly to a λ/2 dipole antenna, if the feeder 22 connected to the feeding point 23 is not made balanced, it is difficult to match an input impedance of the slit antenna 21 to the feeder 22, so that the antenna gain decreases. Therefore, through a balun 25 or the like, the unbalanced feeders 22a and 22b are required to convert to balanced lines 22c and 22d. 
The above described slit antenna is disclosed in Patent document 1.                Patent document 1: Japanese Published Patent Application No. 2003-124738        
However, the above-described slit antenna has become an obstacle to the design of a housing due to the length of an antenna that is λ/2 and too long. For example, in the case where the wave length of a receiving signal is 90 cm, given a contraction rate that is determined based on a relative permeability and a relative permittivity of the high-permeability member covering a slit is TR, the length LT of the slit becomes the length of 45 cm multiplied by the contraction rate TR. Therefore, it is difficult to install the slit antenna in wireless communication apparatuses, particularly, for portable use or of small size such as a mobile telephone unit and PDA, and also it becomes an obstacle to the miniaturization of a housing. Further, requiring a balun or the like to obtain antenna matching also becomes an obstacle to the miniaturization of a housing.